1. Field of the Invention
The present invention concerns a metallic gasket disposed between the joining faces of a cylinder block and a cylinder head, that constitute an internal combustion engine to prevent combustion gas, cooling water, lubricant or the like from leaking therebetween.
2. Description of the Prior Art
In an internal combustion engine, generally referred to as an engine, a gasket is interposed between the joining faces of a cylinder block and a cylinder head which are tightened by a clamping means, such as bolts, to provide a seal therebetween.
In the gasket seal, a seal at the periphery of a combustion chamber, that is, a cylinder bore is particularly important and, if the seal is defective at that portion, the combustion gas in the combustion chamber can not be utilized effectively and results in a lower gas pressure.
Therefore, in an elastic base plate of a metallic gasket, a bead is formed concentrically with a cylinder bore hole and formed parallel to the cylinder bore, so that the surface pressure generated by the previously mentioned bolts between the joining faces of the metallic gasket and the cylinder block or the cylinder head (also referred to as engine parts, respectively) improves the seal at the periphery of the cylinder bore by utilizing the reaction force of the bead that results when the metallic gasket is tightened between the cylinder block and the cylinder head.
In this way, a flat area like other portions of the base plate is formed between the cylinder bore hole and the bead formed at the periphery of the cylinder bore hole. The bead is formed to adjust and compensate for a positional deviation between the punching press used to form the cylinder bore hole and the molding press used to form the bead. However, when the cylinder block and the cylinder head are tightened with the metallic gasket being interposed therebetween, a high surface pressure is generated due to the reaction force of the bead in the vicinity thereof, whereas portions of the cylinder block or the cylinder head in contact with the bead at a position near the cylinder bore hole, namely, portions opposing or in contact with the flat area, deform so as to be separated from each other.
Because of such separating deformation, although a surface pressure can be ensured in the flat area between each of the joining faces of the cylinder block or the cylinder head and the metallic gasket, the tightening force of the cylinder block and the cylinder head to the metallic gasket is lowered and at some time results in a gap between such components. When the engine is operated in such a state, vibration occurs in the metallic gasket from the flat area on which the tightening force is reduced and proceeds to the bead. At the same time, the tightening force to the metallic gasket varies together with the increase or decrease of the cylinder bore pressure during the engine combustion process to induce vibration of the bead. Also, the tightening force to the metallic gasket also varies due to the vibration of the mass or volume change in each of the engine parts caused by their temperature change, so that the vibration or the change of the tightening force may possibly cause fatigue failure, particularly in a portion of the bead which is restricted in movement.
In view of this, it has been proposed, for example, as described in Japanese Utility Model Application Hei 3-56277, to reduce vibration amplitude from a flat area to a bead by securing a shim plate (subplate) to the flat area. In this proposed metallic gasket, as shown in FIG. 12, beads 32 formed on two base plates 30, 31 are inwardly opposed to each other, and a subplate 36 having such a thickness to reduce a gap at a flat area 35 between bead 32 formed on each of the base plates 30, 31 and a cylinder bore hole 33 is disposed at the flat area 35 of one of the base plates (lower base plate 31 in FIG. 12) at the periphery of the cylinder bore hole 33 and the whole periphery is secured, for example, by welding while aligning both cylinder bore holes 33 and 34.
In FIG. 12, reference number 37 denotes a securing line. This suppresses the vibration amplitude starting from the flat area 35 to the bead 32. A reaction force is also generated in the flat area 35 by way of the subplate 36 by increasing the thickness of the subplate 36 so that the tightening force exerted on the bead 36 is dispersed in order to suppress the vibration or fluctuation of the tightening force caused by the increase or the decrease of the cylinder bore pressure generated in the combustion process or by the volume change due to temperature change of the engine parts, thereby suppressing the fatigue failure of the bead 32.
In a case where two or more cylinder bore holes are formed and the distance between adjacent cylinder bore holes decreases, the beads overlap between the adjacent cylinder bore holes. In such a case, it is proposed to join the beads between the cylinder bore holes into a linear configuration and extend it perpendicularly to a line connecting the centers of the adjacent cylinder bore holes (refer to U.S. Pat. No. 4,815,750). This also ensures surface pressure between the cylinder bore holes to which the tightening force of the clamping means, such as bolts, is difficult to apply.
However, with the demand for the engine size reduction along with recent remarkable technical innovations, the distance between the combustion chambers has been reduced and the cylinder bore diameter has been increased so that the distance between the cylinder bores has been greatly decreased. In addition, with the demand for weight reduction, power increase and fuel economy of an engine, use of aluminum material for the engine parts has been developed, auxiliary equipment, such as a super charger, has been attached and the gas mixture has become more lean. With a combination of such factors, the amplitude of vibration generated in the metallic gasket has a tendency to increase.
In addition, as the use of aluminum material for the engine parts has progressed, rigidity of the materials is lowered. Further, although increase of heat capacity of the material caused by the size and weight reduction is coped with, for example, by enlarging the cooling water channel, this reduces engine rigidity even more. In addition, since the cooling water channel is located closer to the cylinder bore, the tightening position by the clamping means, such as the bolts, is removed further from the cylinder bore, and the tendency toward less tightening force of the metallic gasket becomes conspicuous at a position between the cylinder bores.
Taking these various factors into consideration, there is a big difference between the tightening force on the metallic gasket near the clamping means, such as bolts, and the tightening force on a position away from the clamping means, specifically, between the bolts. If the tightening force differs greatly as described above, gap variance is caused around the cylinder bore between the base plates of the metallic gasket or between the base plate and the cylinder block or the cylinder head. If the subplate disposed at the periphery of the cylinder bore hole between the base plates of the metallic gasket is made uniform with respect to the width and the thickness, deformation of the engine parts, such as the cylinder block or the cylinder head, at the periphery of the cylinder bore lose uniformity and, as a result, the cylinder bore circularity is degraded so as to reduce combustion efficiency, increase friction loss, combustion pressure fluctuation and degradation of durability.
The problem is noteworthy, particularly, between the cylinder bores, because there is a limit for the subplate width owing to the narrow space between the cylinder bores, and the difficulty in controlling the reaction force of the metallic gasket between the cylinder bore holes.